[andrewpbrett]

This may be apocryphal but I've heard that "howdy" is essentially a contraction of "how do you do"

Edit: Yep: https://en.wikipedia.org/wiki/Howdy

[TeMPOraL]

I've always assumed that "howdy" was a word from the vernacular of a small regional community in some random US location, that got promoted to mainstream English - individual words from dialects sometimes get picked up and adopted into "canonical" language.